MOVEMENTS AND DEFORMATIONS OF THE EARTH'S BODY. 583 



to account for the crustal shortening of the great mountain-making 

 periods. It is as if the shrinkage stresses accumulated to the full extent 

 of the stress-resisting power of the whole sphere, and then collapsed. 

 It is not safe, however, to give much weight to this coincidence, for 

 higher densities and probably higher resistances to distortion come into 

 play in the deeper horizons. If these resistances are proportional to 

 the higher densities of the interior, the deductions would remain the 

 same. If the effective rigidity of the earth as a whole is that of steel, 

 as deduced by Kehan and Darwin from tidal and other observations, 

 or t\sice that of steel, as inferred by Milne from the transmission of 

 seismic vibrations, the supporting power of the body of the earth 

 dependent on its sphericity would be appreciably higher. 



It would seem clear from the foregoing considerations that some- 

 thing more than the mere crust of the earth has been involved in the 

 great deformations. Indeed it is not clear that the fullest resources 

 of stress-accumulation which the spheroidal form of the earth affords 

 are sufficient to meet the demands of the problem, unless the rigidity 

 of the earth be taken at a much higher value than that of surface-rock, 

 and this is perhaps an additional argument for the high rigidities inferred 

 from tides and seismic waves. 



In \dew of the doubtful competency of even the thickest segments 

 to accumulate the requisite stresses, there is need to consider modes 

 of differential stress- accumulation other than those dependent on 

 sphericity. 



Stress-accumulation independent of sphericity. — The principle of the 

 dome is brought into play whenever an interior shell shrinks away, 

 or tends to shrink away, from an outer one which does not shrink. In 

 this case, there is a free outer surface and a more or less unsupported 

 under surface toward which motion is possible. The dome may, there- 

 fore, }^eld by crusliing or by contortion. The computations given 

 above are for cases of this kind. But where the thickness becomes 

 great and the dome involves a large part or even all of a sector of the 

 earth, freedom of motion beneath is small, and to readjust the matter 

 to a new form, strains must be developed widely throughout the sector, 

 and must involve regions where the pressure is extremely great on all 

 sides, and crushing in the usual sense impossible. Assuming the correct- 

 ness of the modern doctrine that such pressure increases rigidity, instead 

 of the older doctrine that it gives plasticity, it becomes reasonable to 



